A door, an instrument panel, a ceiling material, a rear tray, a pillar, and the like, which are automotive interior materials, generally comprise a molded product and a covering material. As the molded product, a polyolefin molded product is mainly used. On the other hand, a covering material made of a polyurethane foam, a fabric lined with a polyurethane foam, polyolefin, or the like is used as the covering material. The molded product and the covering material are bonded by a pressure bonding method, a vacuum forming method or the like using an adhesive.
A solvent-based one-pack reactive adhesive is used as the adhesive for automotive interior up to date particularly in order to secure creep resistant properties on heating. For example, in Patent Literature 1, a chloroprene rubber-based adhesive comprising modified polychloroprene, a tackifier, and an organic solvent is disclosed. In Patent Literature 2, an adhesive which contains a styrene block copolymer and liquid rubber in combination and do not contain halogen such as chlorine is disclosed. However, the level of environmental requirements is increased, and therefore a switch from the solvent, one-pack, reactive adhesive to a solvent-free adhesive is desired.
Examples of the alternative candidate to render the solvent-based adhesive for automotive interior free from solvents include a hot-melt adhesive. Among the hot-melt adhesives, a reactive hot-melt adhesive advantageously has excellent heat resistance. However, disadvantageously, it is difficult for the reactive hot-melt adhesive to keep a balance between a curing time and stability during application work and storage. In light of bonding properties when an adherend is a polyolefin molded product, a reactive polyolefin adhesive obtained by silane-modifying polyolefin, such as a composition disclosed in Patent Literature 3 is the candidate. However, the reactive polyolefin adhesive emits an organic solvent into an atmosphere by hydrolysis reaction during curing, and therefore there is a fear that the reactive polyolefin adhesive might lack in aptitude as an environment-friendly adhesive.
Among the hot-melt adhesives other than the reactive hot-melt adhesive, a non-reactive, hot-melt adhesive provides no fear that an organic solvent derived from raw materials and a hydrolysis reaction volatilizes from it, and the non-reactive, hot-melt adhesive is thus very greatly expected as the environment-responsive adhesive, and promising. The non-reactive hot-melt adhesive requires no curing time, and has high stability during application work and storage, which is convenient for users. As for a non-reactive adhesive having excellent bonding properties to polyolefin molded products, Patent Literature 4, for example, discloses a use of a hot-melt adhesive in which the surface temperature of a hot-melt adhesive when bonding is set to be equal to or higher than the temperature of the atmosphere when measuring a creep resistant on heating in order to obtain creep resistant properties on heating.